Add compatibility redirects for Mastodon and Soapbox FE

This MR lets Pleroma FE redirects routes used by Mastodon and Soapbox FE so it can support them.

This is useful in case a server admin switches from Mastodon/Soapbox FE to Pleroma FE. Users will have old browser history (and possibly bookmarks) that will continue to work even after the switch.